Reflective liquid crystal display panel and device using same

ABSTRACT

There is disclosed an active matrix reflective liquid crystal display panel on which an active matrix circuit is integrated with peripheral driver circuits. Metal lines in the peripheral driver circuits are formed simultaneously with pixel electrodes. Thus, neither the process sequence nor the structure is complicated.

FIELD OF THE INVENTION

The present invention relates to a reflective liquid crystal displaystructure in which peripheral driver circuits are integrated with othercircuitry.

DESCRIPTION OF THE PRIOR ART

A structure comprising a substrate on which an active matrix circuit andperipheral driver circuits for driving the active matrix circuit are allpacked is known. At least one TFT is disposed at each pixel of theactive matrix circuit. This structure is known as the active matrixdisplay integrated with peripheral driver circuits.

Generally, a peripheral driver circuit is composed of circuits (typifiedby shift registers) and buffer circuits for supplying signals to theactive matrix circuit. However, it is considered that the trend istoward constructing circuits handling image information and varioustiming signals from TFTs and toward integrating these TFTs as peripheraldriver circuits with the active matrix circuit on the same substrate. Inthe past, such circuits have been composed of externally attached ICs.

Essentially, the active matrix circuit comprises source lines and gatelines arranged in rows and columns. TFTs are disposed near theintersections. On the other hand, the peripheral driver circuit is basedon a CMOS circuit. However, it is expected that the circuit will becomemore complex in configuration. In this structure, the use of multilevelwiring is required to reduce the area occupied. However, if anadditional layer is formed to achieve the multilevel wiring, thefabrication process is complicated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a reflective liquidcrystal display panel on which an active matrix circuit is integratedwith peripheral driver circuits having multilevel wiring that can beaccomplished with greater ease than conventional.

The invention disclosed herein is directed to a reflective liquidcrystal display. This kind of liquid crystal display uses metalelectrodes as reflective electrodes. For example, the metal electrodesconsist chiefly of aluminum.

The present invention has been made by taking notice of the material ofthe reflective electrodes. Conducting lines arranged in the peripheraldriver circuits are formed out of the same material as the pixelelectrodes simultaneously with the formation of the pixel electrodes.

In this way, multilevel wiring necessary for the peripheral drivercircuits can be constructed without any additional process step, thoughthe pattern is made more complex.

Since the reflective electrodes can be made of a low-resistivitymaterial such as aluminum, they are preferably used to form conductinglines in the peripheral driver circuits.

In the transmissive liquid crystal display, pixel electrodes are made ofa material having a relatively high resistivity such as ITO. Therefore,the present invention is not adapted to be used in the transmissiveliquid crystal display.

The peripheral driver circuits referred to herein include shift registercircuits and buffer circuits that directly drive the active matrixcircuits. Furthermore, circuits for producing various timing signals,circuits for handling image information, various memory devices, andarithmetic units are included.

The present invention provides a reflective liquid crystal display panelcomprising an active matrix circuit formed on a substrate, peripheraldriver circuits including circuits for driving the active matrixcircuit, and reflective pixel electrodes arranged in rows and columns inthe active matrix circuit. The active matrix circuit and the peripheraldriver circuits are formed on the same substrate. The active matrixcircuit is composed of TFTs. The peripheral driver circuits are alsocomposed of TFTs. The peripheral driver circuits have conducting linesmade of the same material as the reflective pixel electrodes.

In the active matrix circuit of the liquid crystal display of thestructure described above, source lines and the gate lines are arrangedin rows and columns. TFTs are disposed near the intersections of thesesource and gate lines. The drains of the TFTs are disposed at the pixelelectrodes.

Peripheral circuits include circuits composed of shift registercircuits, analog switches, buffers, and so on. This kind of circuit isordinarily referred to as a peripheral driver circuit. Furtherperipheral circuits include oscillator circuits, circuits handling imageinformation, and circuits equipped with memory devices or the like.

It is considered that future trend will be toward adding various otherfunctions to the above-described peripheral circuits. Accordingly, theperipheral circuits referred to herein embrace circuitry having variousfunctions (known as a system-on-panel), as well as circuits for drivingan active matrix circuit.

TFTs can take various forms such as top-gate type, bottom-gate type, andmultigate type in which numerous TFTs are equivalently connected inseries.

Preferably, the material of the reflective electrodes has a highreflectivity and a low resistivity, as typified by silver, aluminum, andsilver-aluminum alloys.

For example, in the case of the VGA standard (640×480 pixels), the frameof image is rewritten or refreshed at a rate of 60 times per second.This requires that the horizontal scanning driver circuit (peripheraldriver circuit on the side of source lines) operate at a rate of640×480×60=18.5 MHz. In the case of the XGA standard (1024×768 pixels),an operating speed of 1024×768×60=47 MHz is necessitated.

In these cases, the resistivity of the conducting lines in theperipheral driver circuits should be made as low as possible. Thepresent invention is quite useful for this purpose.

A specific example in which the conducting lines in peripheral circuitsare made of the same material as the reflective pixel electrodes isshown in FIG. 6. When the pixel electrodes, 141, are formed, theconducting lines, 142, in the peripheral circuits are formed from thesame material as the pixel electrodes.

This is achieved by forming the pattern of the pixel electrodes 141 andthe pattern of the conducting lines 142 simultaneously out of conductingfilm (not shown) that forms the pixel electrodes. This can be checked bytaking an electron microscope image of a cross section of the structure,determining whether the pixel electrodes and the conducting lines arepresent in the common layer, determining whether they are equal in filmthickness, and measuring the doping level to know whether the materialis uniform or not.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 are cross-sectional views illustrating a process sequence forfabricating an LCD (liquid crystal display) panel in accordance with thepresent invention;

FIGS. 9(A)-9(F) are a diagram showing devices equipped with a liquidcrystal panel in accordance with the invention; and

FIG. 10 is a cross-sectional view of another LCD panel in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 6, there is shown a reflective liquid crystal display(LCD) panel in accordance with the present invention. This LCD panel hasa pixel matrix and peripheral circuits. Reflective electrodes 141 aredisposed in the pixel matrix. Conducting lines 142 are formed in theperipheral circuits. The reflective electrodes 141 and the conductinglines 142 are formed simultaneously. This permits the conducting linesof the peripheral circuits to be formed by a common process step.Consequently, the manufacturing process sequence and the structure canbe made simpler.

Since the reflective electrodes can be made of a material having a lowresistivity, the conducting lines of the peripheral driver circuits thatare formed simultaneously with the reflective electrodes can befabricated as low-resistivity lines.

A reflective active matrix liquid crystal display (AMLCD) is fabricatedby a method according to a first embodiment of the present invention.The process sequence for effecting this method is illustrated in FIGS.1-7. N-channel TFTs disposed in a pixel matrix circuit and CMOS circuitsforming peripheral driver circuits are fabricated at the same time.

First, a substrate 101 of glass or quartz is prepared. If the flatnessof the substrate is poor, it is preferable to form a film of siliconoxide or silicon oxynitride on the surface of the substrate.

Generally, a substrate having an insulating surface can be used as theaforementioned substrate. A substrate having an insulating surface canbe a glass substrate, a quartz substrate, a substrate of glass or quartzon which an insulating film of silicon oxide or other material isdeposited, and a semiconductor substrate such as a silicon wafer onwhich an oxide film is deposited.

Then, an amorphous silicon film (not shown) is formed on the surface ofthe substrate to a thickness of 50 nm by low-pressure thermal CVD.

The amorphous silicon film is heat-treated to crystallize the amorphoussilicon film. In this way, a crystalline silicon film is obtained. Thecrystallization may also be effected by irradiation of laser light orother intense light.

Subsequently, the crystalline silicon film is patterned to form islandsof pattern 102, 103, and 104, which will become active layers of TFTs.The island 102 will become the active layer of an N-channel TFT (NTFT)disposed in a pixel matrix circuit. The island 103 will become theactive layer of a P-channel TFT (PTFT) forming a CMOS circuit forming aperipheral driver circuit. The island 104 will become the active layerof an N-channel TFT (NTFT) forming a CMOS circuit forming a peripheraldriver circuit.

The resulting state is shown in FIG. 1. Then, a silicon oxide film 105is formed as a gate insulator film to a thickness of 100 nm by PECVD asshown in FIG. 2.

Thereafter, an aluminum film is formed to a thickness of 400 nm bysputtering. This aluminum film is patterned into stripes 106, 107, and108 which will become the gate electrodes of the TFTs and gateinterconnects extending from the gate electrodes.

An anodic oxide film, 109, 110, and 111, is formed on the aluminumpattern, 106, 107, and 108, to a thickness of 60 nm. The anodic oxidefilm electrically insulates and physically protects the aluminumpattern.

A resist mask (not shown) is deposited on top of the PTFT to cover it.Phosphorus (P) ions are implanted by plasma doping. As a result, thesource region 112, the channel region 113, and the drain region 114 ofthe NTFT arranged in the pixel matrix are formed in a self-alignedmanner. Also, the source region 120, the channel region 119, and thedrain region 118 of the NTFT forming the CMOS circuit of the peripheraldriver circuit are formed in a self-aligned manner.

Then, the resist mask deposited on top of the PTFT is removed. A resistmask is placed on top of the NTFT. Under this condition, boron (B) ionsare lodged by plasma doping. As a result of this process step, thesource region 115, the channel region 116, and the drain region 117 ofthe PTFT forming the CMOS circuit of the peripheral driver circuit areformed in a self-aligned manner.

Subsequently, the resist mask (not shown) is removed. Laser light isdirected at the laminate to improve the crystallinity of the dopedregions and to activate the dopant element. In this manner, a state asshown in FIG. 2 is obtained.

Then, as shown in FIG. 3, a silicon oxide film 121 is formed as aninterlayer dielectric film to a thickness of 500 nm by PECVD. Contactholes are then formed, and a Ti—Al—Ti laminate film issputter-deposited. Each Ti layer of this laminate film is 100 nm, whilethe Al layer is 400 nm. The Ti layers act to improve the electricalcontact with semiconductor or with electrodes.

The Ti—Al—Ti laminate film is patterned to obtain a state as shown inFIG. 3, which portrays a pattern of the laminate film consisting of thetitanium layer 122, the aluminum layer 123, and the titanium layer 124forming the source electrode of the NTFT disposed in the pixel matrix.FIG. 3 also illustrates a pattern of the laminate film consisting of thetitanium layer 125, the aluminum layer 126, and the titanium layer 127forming the drain electrode of the NTFT arranged in the pixel matrix.Furthermore, FIG. 3 depicts a pattern of the laminate film consisting ofthe titanium layer 128, the aluminum layer 129, and the titanium layer130. In addition, FIG. 3 represents a pattern of the laminate filmconsisting of the titanium layer 131, the aluminum layer 132, and thetitanium layer 133 forming the drain electrode of the PTFT of the CMOScircuit. Further, a pattern of the laminate film consisting of thetitanium layer 134, the aluminum layer 135, and the titanium layer 136forming the drain electrode of the NTFT of the CMOS circuit is shown.Additionally, a pattern of the laminate film consisting of the titaniumlayer 131, the aluminum layer 132, and the titanium layer 133constituting the drain electrode of the NTFT of the CMOS circuit isshown. In this way, a state as shown in FIG. 3 is derived.

Then, a silicon nitride film 137 is deposited to a thickness of 50 nm byPECVD. This silicon nitride film 137 forms the dielectric film of anauxiliary capacitor.

Thereafter, a titanium film (not shown) is sputter-deposited to athickness of 150 nm. This film is patterned to form an electrode pattern138 for the auxiliary capacitor. The auxiliary capacitor consists of theelectrode, the titanium electrode 138, and the silicon nitride film 137sandwiched between the titanium electrode 138 and the electrodeconsisting of the titanium layer 122, the aluminum layer 123, and thetitanium layer 124. The silicon nitride film has a large dielectricconstant and can be thinned and so a large capacitance can beaccomplished.

If the liquid crystal panel is used for projection and its diagonaldimension is as small as less than 2 inches, the area of each pixel issmall. Therefore, it is generally difficult to have sufficient auxiliarycapacitance.

This difficulty can be solved by forming the capacitance of thestructure according to the present embodiment. After obtaining the stateshown in FIG. 4, a polyimide resinous film 139 is formed as aninterlayer dielectric film as shown in FIG. 5. The thickness of thepolyimide resinous film 139 is so set that its maximum film thickness is1 μm. Other usable resins include polyamide, polyimidamide, epoxies, andacrylics.

A titanium film having a thickness of 150 nm is formed by sputtering andpatterned to form a pattern 140 shown in FIG. 5. This pattern acts as ashield pattern for preventing electrical interference between its upperlayer (i.e., the pixel electrodes and conducting lines) and its lowerlayer (i.e., TFTs and conducting lines). Also, the portion of the shieldpattern 140 that overlies the driver circuit region shields theperipheral driver circuit against light. In this way, the state shown inFIG. 5 is obtained.

Then, contact holes are created. An aluminum film becoming pixelelectrodes is sputter-deposited to a thickness of 350 nm. Subsequently,this aluminum film is patterned to simultaneously form pixel electrodes141 and conducting lines 142 connecting the peripheral driver circuitand pixel matrix TFTs (FIG. 6).

The conducting lines 142 are formed by making use of the aluminum filmthat constitutes the pixel electrodes 141. Hence, it is not necessary toperform any independent process step. That is, any additional processstep is not necessary to form the conducting lines 142. Instead of thealuminum film, the pixel electrodes 141 may be formed from silver or asilver-aluminum alloy.

After obtaining the state of FIG. 6, an orientation film 143 acting asan orientation film and consisting of polyimide resin is deposited to athickness of 150 nm, as shown in FIG. 7. Thus, one substrate on whichcircuitry consisting of TFTs and undergone orientation processing iscompleted.

After achieving the state of FIG. 7, another substrate made of glass orquartz is prepared and bonded to the substrate (referred to as the TFTsubstrate) shown in FIG. 7. A liquid crystal material is injectedbetween these two substrates. Thus, a reflective AMLCD panel shown inFIG. 8 is obtained.

In FIG. 8, the counter substrate, or the substrate opposite to the TFTsubstrate, is indicated by 147. A counter electrode 146 is made of ITOand located opposite to the pixel electrodes 141 formed on the TFTsubstrate.

A seal material 148 bonds together the substrates 147 and 101 andprevents the liquid crystal material from leaking out. The liquidcrystal material is indicated by 144. Where the LCD panel is of thereflective type, the display is operated in the birefringence mode. Inparticular, the plane of polarization of light propagates through theliquid crystal material layer perpendicularly to the substrate plane,the liquid crystal material consisting of molecules oriented parallel tothe substrates. The plane of polarization changes from verticallypolarized, elliptically polarized, circularly polarized, ellipticallypolarized, and horizontally polarized state in turn.

A second embodiment of the present invention gives examples of a deviceor appliance equipped with an LCD panel in accordance with the presentinvention. That is, these examples are video camera, digital stillcamera, head mount display, car navigational system, personal computer,and portable intelligent terminals (such as mobile computers andportable telephones).

Referring to FIG. 9(A), the body of a mobile computer is indicated by2001. This body 2001 has a display device comprising a reflective LCDpanel 2005. A camera portion 2002 having both an image pickup portion2003 and an operation switch 2004 is attached to the body 2001.

Referring to FIG. 9(B), the body of a head mount display is indicated by2101 and has reflective LCD panels 2102. A band portion 2103 is attachedto the body 2101.

Referring to FIG. 9(C), there is shown a front projection systemcomprising a display body 2201 and a screen 2205 located in front of it.This body 2201 has a light source 2202, a reflective LCD panel 2203, andoptics 2204. Light from the light source 2202 is directed to thereflective LCD panel 2203 via the optics 2204. Then, the LCD panel 2203optically modulates the image. The image is projected onto the screen2205 after magnified by the optics 2204. This type of projection systemneeds the screen 2205 separate from the body 2201.

Referring next to FIG. 9(D), there is shown a mobile telephone whosebody is indicated by 2301. The body 2301 has a speech output portion2302, a speech input portion 2303, a reflective LCD panel 2304, andoperation switches 2305. An antenna 2306 is attached to the body 2301.

Referring to FIG. 9(E), there is shown a video camera whose body isindicated by 2401. This body 2401 has a reflective LCD 2402, a speechinput portion 2403, operation switches 2404, batteries 2405, and animage pickup portion 2406.

Referring to FIG. 9(F), there is shown a rear projection system whosebody is indicated by 2501. The body 2501 has a light source 2502, areflective LCD panel 2503, a polarizing beam splitter 2504, andreflectors 2505, 2506. A screen 2507 is positioned on the body 2501.Light emitted by the light source 2502 is optically modulated by the LCDpanel 2503 via the polarizing beam splitter 2504 and directed at thereflectors 2505 and 2506. Then, the light is reflected by thesereflectors and projected onto the screen 2507 on the body 2501.

A third embodiment of the present invention is based on thefirst-mentioned embodiment shown in FIGS. 1-7 and further characterizedin that gate electrodes consist mainly of silicon. The presentembodiment is schematically shown in FIG. 10, where gate electrodes1001, 1002, and 1003 are made of silicon to which one conductivity typeis imparted. The gate electrodes may also be made of other materialssuch as various silicides and metal materials.

The invention disclosed herein makes it possible to achieve multilevelmetallization with greater ease than conventional, the multilevelmetallization being required in peripheral driver circuits of a devicewhere an active matrix circuit is integrated with the peripheral drivercircuits.

What is claimed is:
 1. A display device comprising: a thin filmtransistor over a substrate; a first conductive film over the substrateand connected with one of a source region and a drain region of the thinfilm transistor, the first conductive film comprising a first conductivelayer, a second conductive layer over the first conductive layer, and athird conductive layer over the second conductive layer; a firstinsulating film including a first contact hole over the first conductivefilm; a second conductive film over the first insulating film; a secondinsulating film including a second contact hole over the secondconductive film; and a third conductive film over the second insulatingfilm and in contact with at least a part of the second conductive layerthrough the first contact hole and the second contact hole, wherein atleast a part of the second contact hole overlaps with the first contacthole.
 2. A device according to claim 1, wherein the third conductivefilm is in contact with a top surface of the second conductive layer. 3.A device according to claim 1, wherein the second conductive filmcomprises titanium.
 4. A device according to claim 1, wherein the thirdconductive film comprises a material selected from the group consistingof silver, aluminum, and silver-aluminum alloy.
 5. A device according toclaim 1, wherein the first insulating film comprises silicon nitride ora resin.
 6. A device according to claim 1, wherein the first insulatingfilm comprises a material selected from the group consisting ofpolyimide, polyamide, polyimidamide, epoxies and acrylics.
 7. A deviceaccording to claim 1, further comprising a pixel electrode over thesecond insulating film wherein the third conductive film comprises asame material as the pixel electrode.
 8. A device according to claim 1,wherein the first conductive layer comprises titanium, and the secondconductive layer comprises aluminum, and the third conductive layercomprises titanium.
 9. A device according to claim 1, wherein thedisplay device is incorporated into one selected from the groupconsisting of a mobile computer, a head mount display, a frontprojection system, a mobile telephone, a camera and a rear projectionsystem.
 10. A display device comprising: a thin film transistor over asubstrate; a first conductive film over the substrate and connected withone of a source region and a drain region of the thin film transistor,the first conductive film comprising a first conductive layer, a secondconductive layer over the first conductive layer, and a third conductivelayer over the second conductive layer; a second conductive film overthe substrate; an insulating film including a contact hole over thefirst conductive film and the second conductive film; a third conductivefilm over the insulating film and in contact with at least a part of thesecond conductive layer through the contact hole, wherein at least apart of the third conductive film overlaps with the second conductivefilm.
 11. A device according to claim 10, wherein the third conductivefilm is in contact with a top surface of the second conductive layer.12. A device according to claim 10, wherein the second conductive filmcomprises titanium.
 13. A device according to claim 10, wherein thethird conductive film comprises a material selected from the groupconsisting of silver, aluminum, and silver-aluminum alloy.
 14. A deviceaccording to claim 10, further comprising a pixel electrode over theinsulating film wherein the third conductive film comprises a samematerial as the pixel electrode.
 15. A device according to claim 10,wherein the first conductive layer comprises titanium, and the secondconductive layer comprises aluminum, and the third conductive layercomprises titanium.
 16. A device according to claim 10, wherein thedisplay device is incorporated into one selected from the groupconsisting of a mobile computer, a head mount display, a frontprojection system, a mobile telephone, a camera and a rear projectionsystem.
 17. A display device comprising: a thin film transistor over asubstrate; a first conductive film over the substrate and connected withone of a source region and a drain region of the thin film transistor,the first conductive film comprising a first conductive layer, a secondconductive layer over the first conductive layer, and a third conductivelayer over the second conductive layer; a first insulating filmincluding a first contact hole over the first conductive film; a secondconductive film over the first insulating film; a second insulating filmincluding a second contact hole over the second conductive film; and athird conductive film over the second insulating film and in contactwith at least a part of the second conductive layer through the firstcontact hole and the second contact hole, wherein the second conductivefilm is not in contact with the first conductive film and the thirdconductive film.
 18. A device according to claim 17, wherein the thirdconductive film is in contact with a top surface of the secondconductive layer.
 19. A device according to claim 17, wherein the secondconductive film comprises titanium.
 20. A device according to claim 17,wherein the third conductive film comprises a material selected from thegroup consisting of silver, aluminum, and silver-aluminum alloy.
 21. Adevice according to claim 17, wherein the first insulating filmcomprises silicon nitride or a resin.
 22. A device according to claim17, wherein the first insulating film comprises a material selected fromthe group consisting of polyimide, polyamide, polyimidamide, epoxies andacrylics.
 23. A device according to claim 17, further comprising a pixelelectrode over the second insulating film wherein the third conductivefilm comprises a same material as the pixel electrode.
 24. A deviceaccording to claim 17, wherein the first conductive layer comprisestitanium, and the second conductive layer comprises aluminum, and thethird conductive layer comprises titanium.
 25. A device according toclaim 17, wherein the display device is incorporated into one selectedfrom the group consisting of a mobile computer, a head mount display, afront projection system, a mobile telephone, a camera and a rearprojection system.